Transmission line hangers are used to support transmission lines. In most uses, transmission lines are attached to a plurality of supporting structures along most of their lengths by a plurality of transmission line hangers. The most extensive use of hangers is in connection with coaxial cables, such as those disclosed in U.S. Pat. No. 5,334,051 (Devine et al.) and U.S. Pat. No. 5,167,533 (Rauwolf). Coaxial cables are extensively used for carrying electromagnetic signals.
Transmission line hangers are used for supporting coaxial cables mounted on tall towers and along horizontal bridges. Hanger failure presents both functional and safety issues of a very serious nature. A variety of mounting and support means for coaxial cables have been disclosed in the prior art. For example, U.S. Pat. No. 4,813,639 (Midkiff et al.) discloses a cluster mounting system for supporting two or more coaxial cables.
Hanger stress is a continuing problem for prior cable hangers. Many cable hangers are made of plastic. However, prior plastic cable hangers generally do not have the ability to accommodate a range of transmission line diameters. Prior designs include two identical and opposing plastic hanger halves that are joined at a mating plane. In these prior designs, as the cable diameter exceeds the inside diameter of the assembled plastic hanger, the hanger halves are forced apart at the mating plane. As the cable diameter increases, the stresses in the plastic hanger increase. Similar stress are created when the cable is bent using the hanger as a fulcrum. These stresses cause the plastic to deform over time. Contributing to the stresses causing deformation are the imbedded stresses that are inherent in the molding process itself. These imbedded stresses tend to work in the same direction as the stresses caused by deformation. All of the combined stresses force these prior hangers open at the mating plane which cause the hangers to lose their grip on the subject cable. The adverse effect of all of these stresses is amplified and accelerated by environmental conditions and changes such as heat, cold and humidity. This is significant because cable hangers are typically used outdoors in such environmental conditions.
Longitudinal movement of the cable with respect to the hanger is another continuing problem. Typically, the cable jacket is made of plastic. The polyethylene typically used for cable jackets is a material that has an inherent lubricant quality. In addition, the cable jacket can "cold flow" which reduces the holding force of the cable hanger over time, resulting in cable slippage.
U.S. Pat. No. 5,794,897 (Jobin et al.) discloses a cable hanger that addresses some of the above problems. This hanger clamps two parallel cables at once and includes two opposing elements each having two posts and two openings for aligning the two elements when they are placed around the transmission lines. Once aligned, the two elements are brought toward each other until they snap together to form a clamping structure which loosely grips the transmission lines. The clamping structure can then be slid to the desired location on the transmission lines. A bolt, screw or similar threaded member is then inserted through the aligned bolt or screw openings in the two elements and through the supporting structure. Finally, the bolt, screw or similar threaded member is tightened to securely clamp the opposing elements to the transmission lines and to attach the resulting hanger to the supporting structure. A plurality of randomly spaced notches extend along the inside of the elements to firmly clamp the transmission lines and reduce cable slippage.
FIG. 1 illustrates one half of a representative prior art cable hanger that includes a plurality of randomly spaced ribs or notches 5 that protrude from the interior surface of each cable hanger halve to better clamp the exterior of the transmission line. However, the ribs 5 in these prior hangers are not spaced to fit into the recesses of the corrugated cable. Instead, the ribs 5 are spaced a random distance apart along the interior surface of the cable hanger. Therefore, one or more of these ribs 5 typically fall on a corrugation crest and thus force the hanger halves apart. Moreover, ribs that are randomly spaced apart along the interior surface of a cable hanger limit the diameter range such a hanger can accommodate.
Prior hanger designs can only effectively accommodate one diameter transmission line. In these designs, the inner hanger diameter is sized for the smallest cable diameter the hanger is designed to hold. A cable approximately two percent larger than the inner hanger diameter will deflect the two hanger halves enough to cause hanger deformation. Since most of the prior designs do not include a limiting device (such as a snap clasp), the plastic will eventually distort beyond the limits of its basic configuration thus causing the hanger to lose its hold on the cable thereafter.
Furthermore, the installation of prior transmission line hangers is often a difficult and time-consuming operation because of the large amount of auxiliary hardware required to install these prior hangers. Therefore, there is a continuing need for effective, inexpensive hangers and for improved methods of installing them. The present invention includes several unique features that address these continuing needs by providing a transmission line hanger that minimizes the clamping stress between the hanger and the transmission line, inhibits hanger deformation, reduces longitudinal movement of the transmission line with respect to the hanger, allows a range of corrugated transmission line diameters to be accommodated, and facilitates easy installation.